Be positive, but not in 2017

Algebra Level 4

Find the minimum value of x + 1 + x + 5 + x + 14 + x + 97 + x + 1920 \left| x+1 \right| +\left| x+5 \right| +\left| x+14 \right| +\left| x+97 \right| +\left| x+1920 \right| .


The answer is 2011.

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1 solution

Lemma: If x , y x,y are real numbers, we have: x + y x + y \left|x\right|+\left|y\right|\ge\left|x+y\right| . The equation holds iff x y 0 xy\ge0 .

Applying this lemma, we have:

x + 1 + x + 5 + x + 14 + x + 97 + x + 1920 \quad \left| x+1 \right| +\left| x+5 \right| +\left| x+14 \right| +\left| x+97 \right| +\left| x+1920 \right|

= x 1 + x + 1920 + x 5 + x + 97 + x + 14 =\left| -x-1 \right| +\left| x+1920 \right|+\left| -x-5 \right| +\left| x+97 \right|+\left| x+14 \right|

x 1 + x + 1920 + x 5 + x + 97 + 0 \ge\left|-x-1+x+1920\right|+\left|-x-5+x+97\right|+0

= 2011 =2011

The equality holds iff x = 14 x=-14 .

Good solution!

Steven Jim - 4 years ago

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